Field of the Invention
The present invention relates to an image pickup apparatus such as a digital camera, a method of detecting a change in the amount of light during image pickup, and a storage medium, and in particular to a technique for reducing unevenness of exposure arising from a change in the amount of light from an artificial light source during image pickup under the artificial light source.
Description of the Related Art
Conventionally, when images are picked up using an image pickup apparatus such as a digital camera under an artificial light source with varying amounts of light, the effects of flicker on picked-up images are reduced by synchronizing shutter speed during light exposure to a cycle of change in the amount of light so that amounts of light exposure in respective scanning lines of an image pickup device can be uniform. In the following description, as deemed appropriate, an artificial light source with varying amounts of light is referred to as a flickering light source, and a change in the amount of light is referred to as flicker.
A variety of methods for accurately detecting flicker when picking up an image have been proposed. For example, there has been proposed a method that calculates a value of line lightness by summing pixel levels in each scanning line within one frame, extracts a variation cycle of the calculated value of line lightness in a vertical scanning direction, and when the extracted variation cycle falls inside a predetermined frequency range, determines that there is flicker (see Japanese Laid-Open Patent Publication (Kokai) No. 2004-260574).
However, even when it is possible to accurately detect flicker, frame speed during continuous shooting cannot be increased if shutter speed in light exposure is synchronized to a cycle of change in the amount of light of flicker. Also, frame speed cannot be increased if the presence or absence of flicker is determined each time a still image of one frame is picked up during continuous shooting. On the other hand, as the resolution of image pickup devices is increasing lately, it is becoming increasingly common to pick up images with a high shutter speed of, for example, 1/4000 second even under a flickering light source, but the above publication of the prior art does not mention a technique for applying flicker detection results to continuous shooting at high shutter speed.